1. Field of the Invention
This invention relates to radiant energy sensors, particularly integrated circuit semiconductor photodiode arrays wherein individual diodes serve as detectors of individual picture elements within a thermal or far-infrared image.
2. Description of the Prior Art
Of the numerous devices that have been used for the detection of radiant electromagnetic energy the most promising at the present time, particularly for the far-infrared, are semiconductor photodiodes. These can operate in the photovoltaic mode with no external bias, but they perform better when back-biased to form a depletion region close to the pn junction. For example, semiconductor materials consisting of certain elements from Groups III A and V A or preferably Groups VI A and II B provide the required bandgap energies for infrared detectors when doped with certain elements from Group V A (p-type) or Group III A (n-type).
To form a photodiode array it has been the practice to deposit a thin two layer p-n semiconductor photodiode having a large surface area on a thick substrate of suitable characteristics. This area is then divided into smaller areas which form picture elements or pixels, e.g. one such array has 512 rows and 512 columns with more than 250,000 elements in a square array. The pixels were formed by delineation of the p-n layer and then each was contacted by depositing a small metal electrode in the middle of each pixel on one layer of the diode, and a metal common return contact on the other layer. The substrate is formed of a material transparent to the far-infrared radiation, which passes therethrough to be absorbed in the adjacent photocurrent generating layer of the diode.
The theory of operation was that the signal obtained by loading a specific probe contact was determined only by those photocurrents nearer to it then to another adjacent contact. This turned out not to be the case and to reduce the effect of other photocurrents grooves were cut through the cap contact layer of the diode on which the probe contacts were deposited. These grooves, which extend from one edge to the opposite edge of the contact layer and between the rows and columns of contacts, created separate islands mostly above the pn boundary and somewhat reduced the cross-talk between adjacent pixel detectors. The photocurrent generating layer was kept substantially intact to form with the metal return contact the return current electrode for all of the detectors. Unfortunately, it is in this layer that most of the cross-talk between photocurrents takes place and is little affected by the isolation techniques described above.